Electronic locking devices for motor vehicles are known in many forms. Vehicles manufactured at present often have a locking device in which a base unit, preferably a transceiving device, is provided in the vehicle and the vehicle driver carries with him a conventional vehicle key which additionally has at least one manually triggerable transmitter. The transmitter is usually activated by operating a pushbutton or by the triggering of a snap-action mechanism in which the key is rotated from a retracted rest position into a lockable position.
In the simplest case, the transmitter, after having been activated, transmits a code, for example a relevant bit sequence, to the base unit in the motor vehicle which compares the received bit sequence with a stored bit sequence and, if they agree, triggers an operation of the lock. However, this technique entails the risk that the transmitted key, i.e. the transmitted bit sequence, is “monitored” and unauthorized persons can attain access to the vehicle.
For this reason, electronic locking systems have been developed in which the code triggering an action changes in each case or at predetermined time intervals. For example, it is known that the base unit in the motor vehicle transmits a random bit sequence to the key unit on request, wherein the key unit, after receiving the random bit sequence, codes the latter by using a predetermined rule and sends it back to the base unit. The base unit also knows the coding rule and compares the received coded bit sequence with the expected result or, respectively, decodes the bit sequence by using a corresponding decoding rule and compares the decoded bit sequence received with the bit sequence sent out. If they match, the corresponding action, for example the triggering or the enabling of a locking process or unlocking process is then effected again.
A disadvantage of this known system is the mandatory presence of a transmitter in the electronic key unit and the associated expenditure and the energy requirement that depends on the transmitter.
In recent times, the development tends to construct an electronic locking device in such a manner that the desired or required actions are automatically triggered or at least enabled at the motor vehicle. For example, when the vehicle driver approaches the motor vehicle, at least the relevant vehicle door should be unlocked after he has come closer than a predetermined minimum distance.
German Patent No. 35 32 156-C2 discloses a locking device for motor vehicles in which a corresponding, continuously running oscillator with accurate timing is provided both in the base unit and in the key unit. The two oscillators are synchronized with one another at a predetermined counting rate to an associated generator of sequences of numbers. Both generators contain a predetermined sequence of numbers which they continue to count, pulse by pulse, at matching rates and output the current count to the input of a computer or, respectively, a coding unit at the same time. At another input of the computer, a read-only memory is connected which informs the computer of an invariable characteristic code number of the key unit (identification code). Both the identification code and the count are combined in both the key unit and the base unit to form a combination code in accordance with a predetermined matching algorithm. The combination code is transmitted by the key unit to the base unit, wherein the base unit triggers a control pulse which operates various lock processes when codes match.
Although this creates a burglary-proof locking device, this device also requires an active transmitter in the key unit, with the corresponding energy consumption.
Various techniques are known to keep the energy consumption in the base unit and/or the key unit as low as possible. For example, it is known to activate the receiver in the key unit continuously and to cause the base unit in the motor vehicle to output a corresponding signal only when a door handle of the motor vehicle is mechanically operated, which signal then triggers a corresponding interaction between the key unit and the base unit that leads to the unlocking of the lock. Although this makes it possible to achieve a drastic reduction in the energy consumption in the base unit, the key unit must be continuously ready for reception, or in sufficiently short time intervals. This leads to a corresponding energy consumption in the key unit.
Furthermore, it is known also to clock the key unit so that the information triggering the desired action is only sent at particular time intervals. Although this leads to a reduction in the energy consumption at the key unit end, the response time is extended until the desired action is triggered.
Finally, it is known in transmission engineering to modulate a radio-frequency carrier virtually without power consumption, by changing the matching of an antenna by a modulating signal. This results in a modulated signal reflected or backscattered by the antenna. This method is also called backscatter modulation.